There are a number of methods for squelching or muting radio receivers so as to avoid the disturbance and fatigue caused by the reception of noise during idle radio circuit periods. For example, and improved squelch system was described in my U.S. Pat No. Re. 27,202 and further improvement were disclosed in patent application Ser. No. 693,716 filed on June 7,1976.
Squelch circuits commonly utilize a "hang time" of 0.1 to 5.0 seconds so that if there is a short interruption in the signal flow the communications circuit continues to operate. The term "hang time" as used in this specification and certain of the appended claims is defined as the time a squelch circuit requires to revert to the squelched state at the end of a transmission. It is important to incorporate circuitry in squelch systems which provides hang time so as to avoid annoying interruptions of speech reception during short fades or other breaks in reception. Such interruption can cause loss of speech information and introduce clicks. Even longer hang times may be used in suppressed carrier SSB transmission systems where pauses in speech produce corresponding pauses in transmission. In mobile communications system, no matter what type of modulation is used, the signal strength can suddenly decrease and cause the squelch circuit to mute the audio system. In such cases, a long squelch hang time can avoid lengthening interruptions in reception.
One disadvantage in using long hang times is that a noise "burst" or noise "tail" may be heard. This is particularly disturbing if FM systems incorporating limiters are used because the limiters fill the system with noise when the signals are absent. It is also disturbing for other types of communications systems where the hang time is long enough for the AVC circuit to recover, increasing the receiver's gain before the system is squelched. Also, some transmitters emit high noise levels as they are keyed off, which is disturbing to the listener.
One procedure for easing this noise burst problem is to shorten the hang time to a point where the noise burst is of such short duration that it is not disturbing. However, if the hang time is so reduced to say less than one-tenth of a second, this system will be sensitive to signal dropout problems mentioned above.
The Di Toro U.S. Pat No. 3,081,457, Adams et al U.S. Pat. No. 3,195,048, Rosenberg et al U.S. Pat. No. 3,305,782 and Hepp U.S. Pat. No. 3,739,285 U.S. patents all disclose means for utilizing storage means for improving the signal-to-noise ratio of radio systems. Of particular interest here is Winterbottom's U.S. Pat. No. 3,397,401 which describes various methods of using delay means for the removal of noise bursts at the end of signal transmission.